Nembrotha spec. nov.
Nembrotha rutilans Debelius & Kuiter, 2007: 62, lowest left photo only (non rutilans Pruvot-Fol, 1931).
Nembrotha chamberlaini Gosliner et al., 2008: 116, right photo only (non chamberlaini Gosliner & Behrens, 1997).
Material.— RMNH.MOL.131696, Bunaken, Manado, North Sulawesi, 5-15 m depth, xi.1990, 20 × 10 mm pres.; leg. and photos H. Debelius.
Description.— The single specimen is well preserved, and the brown patches remain on the dorsum. The photographs depict two large patches of chocolate brown in front of and behind the gills (pl. 2 fig. 1), extending the length and breadth of the body and meeting on either side of the gills; there was no gold line surrounding the brown dorsal patches. The body colour was opaque white, with a faint yellow line only along the violet margin of the foot. The three large gills were red distally and white proximally: the common basal stump had an ochre patch extending onto the notum. Other ochre patches were also present at the base of the rhinophores and near the tip of the tail. None of these had a gold border although they had golden flecks within. The rhinophores had dark reddish-brown lamellae, a white tip, and a paler stalk. Ventrally, the foot corners, head, oral tentacles, and tip of the tail were violet (pl. 2 fig. 2).
In the preserved specimen, the blue line on the head and oral tentacles is still present as a dense band and internally, there is a blue band remaining on the oral tube. The radula was examined and has the formula > 13 × 188.8.131.52.7-8. It is deformed, missing all but one marginal tooth on the left side. The rachidian bears five regular denticles on the cusp and is broadest below these, tapering to the slightly rounded base (fig. 2). Unlike those of Nembrotha lineolata (p. 909), the denticles are approximately equal in size and are not joined at their bases. The rachidian teeth are broader than they are high. The first lateral is typical for the genus, very large with a broad base and hooked cusp; the marginal teeth are small rectilinear plates decreasing in size along the row.
Remarks.— The most similar species is Nembrotha chamberlaini Gosliner & Behrens, 1997, originally described and subsequently recorded only from the Philippines (Pola et al., 2008); two individuals were photographed in Bohol, Philippines (pl. 2 fig. 3; pers. comm. J. Hinterkircher, ix.2003 and iii.2004). There are slight variations in the tone of colours but the pattern remains very constant: N. chamberlaini has uniformly red gills and rhinophores with a contiguous intense red patch at the base of each, and a red patch is also present at the end of the tail; these red patches often have a yellow ring around them. The large chocolate brown dorsal patch is also surrounded by a golden yellow line. In the new species described here (and many other photographs available on NudiPixel and SeaSlugForum) the red patches are replaced by ochre and the gills and rhinophores are bicoloured red and white, not uniformly deep red. The large brown dorsal patch of N. chamberlaini is edged in yellow in the original description and other photographs but not in this species. The radula of N. chamberlaini is similar, perhaps longer, and the rachidian of a 62 mm preserved specimen was more asymmetrical than in this small 20 mm preserved specimen.
Photographs of this species on the internet are now filed under Nembrotha purpureolineata O’Donoghue, 1924, because most were originally recognised as different and assigned to N. rutilans (Pruvot-Fol, 1931). When N. rutilans was synonymised with N. purpureolineata by Pola et al. (2008), the photos were simply reassigned despite the differences evidenced by their original designations. (The synonymy of rutilans with livingstonei Allan, 1933, in Gosliner et al. (2008) is erroneous. However, this error serves the point that these forms can be separated, and have been so in the past.) Nembrotha purpureolineata is indeed similar but never has red patches on the dorsum at the bases of the gills and rhinophores nor on the tip of the tail; additionally, it is only found in Australia. Its brown lines may merge to form smaller and more striped patches. If anything, purpureolineata is most easily confused with N. lineolata (p. 909), which has brown lines on the dorsum almost identical to the thin-lined forms of N. purpureolineata. However, despite the lack of external and internal differences, Pola et al. (2008) maintain these two as distinct species. This specimen from Sulawesi is identified as Nembrotha spec. nov. because of its pattern of ochre patches on the mantle at the bases of the bicoloured gills and rhinophores and on the tail, not surrounded by a gold line. This is a constant feature, as evidenced by many photographs from Indonesia on NudiPixel, MedSlugs, and the SeaSlugForum depicting animals identical in colour pattern to Nembrotha spec. nov.; the species shows no variation in colour pattern towards either chamberlaini or purpureolineata. Significantly, all individuals appear to have been photographed only in Indonesian waters.
Both chamberlaini and purpureolineata are confused, but if the presence or absence of the three red patches is used to distinguish them, they separate neatly into three species. Nembrotha purpureolineata has a streakier pattern with pale bases to the bicoloured gills and rhinophores, and is recorded only from northern Australia. Nembrotha chamberlaini has large brown patches surrounded by gold, uniformly red gills and rhinophores, red dorsal patches at the bases of the gills and rhinophores, and is recorded from the Philippines, Indonesia, and Malaysia. Nembrotha spec. nov. is ochre with bicoloured rhinophores and gills, no red patches, and recorded only from Indonesia. It is more prudent at this stage to retain the three clearly distinguishable species, based on external colouration and geographical distribution, than to combine them into one widely distributed and highly variable species (with four forms since we must also include N. lineolata). It has been stated by most authors that the only consistent feature separating species of Nembrotha is colour pattern; if this gold patterned species is assigned to N. chamberlaini, then all species will have to be re-assessed with the same criteria: the end result would be very few species with many colour forms.